Foveation and Acuity Deficits in Patients With Idiopathic Infantile Nystagmus Syndrome Across a Wide Age Range

Abstract

Purpose: : Previously we introduced the Nystagmus Optimal Foveation Function (NOFF) (Felius et al, 2008 ARVO) as an objective ocular motor outcome measure for infants and young children in whom determining the Expanded Nystagmus Acuity Function (NAFX) may not be feasible. The NAFX (Dell’Osso et al, 2002 Doc Ophthalmol) is an established measure of foveation, predictive of visual acuity in patients with idiopathic infantile nystagmus syndrome (IINS). Both NAFX and NOFF are based on simultaneous position and velocity criteria that define ‘foveation’, but the NOFF allows for within-session positional changes. Here we validate the NOFF against the NAFX in older patients and study the relationship with visual acuity in IINS from infancy to adulthood.

Methods: : Calibrated horizontal eye movement data from 20 patients with IINS (age 6-27 years) were analyzed using both NOFF and NAFX algorithms, and the results compared to binocular visual acuity from the same visit. The NOFF and age-corrected visual acuity data were combined with data from 32 infants and young children (age 5 months-6 years) with IINS.

Results: : In the 6-27 year age group, the NOFF (mean±SD=-1.5±1.7 logits) correlated with the NAFX (0.43±0.24 logMAR) (r²=0.94; P<0.001). The acuity values predicted by the NAFX algorithm correlated with measured visual acuity (0.36±0.25 logMAR) (r²=0.37; P=0.004). For the combined group of 52 patients, age-corrected visual acuity was associated with the NOFF and fit by a exponential model as previously described, explaining 35% of the variance (P<0.001).

Conclusions: : The excellent correlation with the NAFX in older children and adults validates the NOFF as a measure of foveation. The strong relationship between NOFF and visual acuity in IINS explained a similar amount of variance as in the relationship between NAFX and visual acuity in older patients. These results confirm the association between foveation statistics and binocular visual acuity in IINS and extend it to a very young age range. The finding that only 1/3 of the variance in visual acuity is explained by either NOFF or NAFX may be due to noise, the somewhat limited range of this cohort, and/or it may indicate that factors other than foveation statistics play a role.

Keywords: infant vision • eye movements • visual acuity